The present invention relates to a scanning optical system employed, for example, in a laser beam printer. More specifically, the present invention relates to a scanning optical system having an automatic power control (APC) circuit for controlling the output power of a laser diode provided in the scanning optical system.
It sometimes becomes necessary to change an output power of a laser diode in a laser beam printer. For example, in order to change an image forming speed (i.e., a scanning speed) of the laser beam, it also becomes necessary to change the output power of laser beam on a surface to be scanned (e.g., the surface of a photoconductive drum).
In another case, in a color laser beam printer employing a plurality of scanning optical systems and a plurality of photoconductive drums, since all the photoconductive drums may not have the same sensitivity to light, it may become necessary to adjust the output power of laser diodes corresponding to the photoconductive drums, respectively.
Conventionally, for controlling the output power of the laser diode(s), a scanning optical system having an automatic power control (APC) function has been widely used. An example of an output power switching apparatus which switches the output power of the laser diode by performing the APC function is disclosed in Japanese Provisional Patent Publication HEI 6-164037.
The light power switching apparatus described in the above publication is schematically shown in FIG. 1. In the power switching apparatus shown in FIG. 1 includes a laser diode LD, which emits a laser beam, a driving current generating circuit 1 which receives an APC signal and generates a driving current, which causes the laser diode LD to emit a laser beam, a photodiode PD, which receives the laser beam (a back beam) and generates an electrical current corresponding to the intensity of the received laser beam. The electrical current generated by the photodiode PD flows through a resistor connected in series with the photodiode PD, and is converted into a detection voltage Vs. The power switching apparatus further includes a comparator 3 which compares the detection voltage Vs corresponding to the electrical current generated by the photodiode PD with a reference voltage Vref and then outputs a result of the comparison to the driving current generating circuit 1, and a variable-type reference voltage generating circuit 4 which changes the reference voltage Vref based on data externally inputted. Further, the light power switching apparatus includes a switching circuit 2 which ON/OFF controls the driving current supplied from the driving current generating circuit 1 to the laser diode LD in accordance with an image signal.
The reference voltage generating circuit 4 generates the reference voltage Vref responsive to the externally input data.
The data externally input to the reference voltage generating circuit 4 is well correlated with a required intensity of the laser beam on the surface of the photoconductive drum. The externally input data represents the required intensity of the laser beam, and therefore, the reference voltage generating circuit 4 generates the reference voltage Vref which corresponds to the required intensity of the laser beam on the surface of the photoconductive drum.
The driving current generating circuit 1 varies the driving current supplied to the laser diode LD in accordance with the comparison result of the comparator 3 so that the detection voltage Vs coincides with the reference voltage Vref. As a result, the intensity of the laser beam on the photoconductive drum coincides with the required intensity.
In general, the photoconductive drum, the laser diode LD, the photodiode PD and a polygonal mirror (not shown in FIG. 1), and a circuit including the comparator 3, the driving current generating circuit 1 and the switching circuit 2 are arranged integrally in a scanning optical unit, which is placed in a housing of the laser beam printer. The reference voltage generating circuit 4 is arranged in the housing as a control unit, separately from the scanning optical unit.
One of the reasons for adopting such a configuration is to facilitate maintenance of the laser beam printer. The scanning optical unit relatively easily degrades with aging, while, the control unit hardly deteriorates with aging. Therefore, in most cases, for the purpose of the maintenance, only the scanning optical unit is replaced.
However, if the scanning optical unit and the control unit are configured as separated units, it becomes necessary to use a cable in order to supply the reference voltage Vref from the reference voltage generating circuit 4 to the comparator 3. The scanning optical unit and the control unit are often spaced apart from each other. In such a case, the cable, which transmits the reference voltage Vref, should be made relatively long. In this case, since the reference voltage Vref is transmitted as an analog signal, a noise may be superimposed on the reference voltage Vref within the cable. Such a noise causes a small variation of the reference voltage Vref, which results in a variation in driving current that is generated by the driving current generating circuit 1. As a result, the output power of the laser diode LD may change significantly. In particular, if the reference voltage is lifted to a relatively high voltage due to the noise, then a significantly strong current may be supplied to the laser diode LD, and may damage the laser diode LD.
In order to prevent the effect of the noise on the reference voltage Vref, the reference voltage generating circuit 4 may preferably be arranged in the scanning optical unit. Then, since a path, across which the reference voltage Vref is transmitted to the comparator 3, is located on the circuit board in the scanning optical unit, the noise on the reference voltage Vref will be suppressed sufficiently.
However, since the reference voltage generating circuit 4 is generally includes a DIA converter which converts digital data corresponding to the reference voltage Vref to an analog signal, the reference voltage generating circuit 4 is relatively large and expensive.
In view of downsizing of a laser beam printer, it is preferable to downsize the scanning optical unit. In particular, a color laser beam printer, which has a plurality of photoconductive drums and a plurality of scanning optical units, requires that the size of the scanning optical units is reduced. However, if the reference voltage generating circuit 4 is included in the scanning optical unit, then the scanning optical unit can not be downsized.
In addition, if the reference voltage generating circuit 4 is provided in the scanning optical unit, the scanning optical unit becomes expensive. Since the scanning optical unit is to be replaced when maintenance is carried out, the maintenance cost also becomes expensive.
It is therefore an object of the invention to provide an improved scanning optical device having an APC (automatic power control) circuit for controlling an output power of a laser diode, with which increase of manufacturing/maintenance costs of the scanning optical unit is avoidable, without being upsized.
For the object, according to the present invention, there is provided a scanning optical device having an automatic power control circuit for controlling an output power of a laser diode. The automatic power control circuit is provided with a detecting system that detects an output power of the laser diode and outputs a detection value corresponding to the detected output power of the laser diode, and a reference value setting system that outputs a reference value, the reference value setting system including a first circuit that outputs a first signal having a predetermined waveform, the waveform varying as time elapses, and a sample-and-hold circuit that samples and holds the first signal. Further, the scanning optical device is provided with a triggering system that transmits a trigger signal, the sample-and-hold circuit sampling and holding the first signal in response to the trigger signal, the sampled and held value of the first signal being output from the reference value setting system as the reference value. The automatic power control circuit is further provided with a comparing system that compares the detection value with the reference value, and outputs a signal representing a difference between the detection value and the reference value, and a driving current controlling system that controls a driving current flowing through the laser diode in accordance with the signal output by the comparing system to vary the output power of the laser diode so that the detection value coincides with the reference value.
With this configuration, the automatic power control circuit is capable of controlling an output power of a laser diode accurately, with suppressing the manufacturing/maintenance costs of the scanning optical unit. Further, since the circuits are relatively simple, the scanning optical unit can be downsized.
Optionally, the automatic power control circuit may further include an APC controlling system that transmits an APC signal to the comparing system at a predetermined timing, the comparing system comparing the detection value with the reference value upon reception of the APC signal.
Further optionally, the triggering system may configured to determine a timing at which the trigger signal is output in accordance with at least one of a scanning speed, a sensitivity of a surface to be scanned and a resolution of an image to be formed.
In particular, the first circuit may include a saw-tooth wave generating circuit, a waveform of a signal output by the saw-tooth wave generating circuit linearly hanging as time elapses.
In a particular case, an amplitude of the signal output by the saw-tooth wave generating circuit may increase as time elapses.
Still optionally, the laser beam emitted by the laser diode is periodically scanned at a predetermined cycle, and the saw-tooth wave generating circuit generates the signal having a saw-tooth shaped waveform synchronously with the periodical scanning of the laser beam.
Further optionally, the amplitude of the signal output by the saw-tooth wave generating circuit may increase from its minimum value at every scanning of the laser beam.
Preferably, the amplitude of the signal output by the saw-tooth wave generating circuit may vary between the minimum value and a predetermined maximum value.
Optionally, the scanning optical device includes a scanning optical unit that deflects the laser beam emitted by the laser diode to scan on a surface to be scanned, and the scanning optical unit is integrally constructed with the automatic power control circuit. In this case, the triggering system can be constructed separate from the scanning optical unit, and the triggering system and the scanning optical unit are connected with at least a cable for transmitting the triggering signal.
Further optionally, the laser beam emitted by the laser diode periodically scans at a predetermined cycle, the saw-tooth wave generating circuit generating the signal having the saw-tooth shaped waveform synchronously with the periodical scanning of the laser beam.
Still optionally, the scanning optical unit may include a synchronizing signal generating system that generates a synchronizing signal which is output synchronously with every scanning of the laser beam, the saw-tooth wave generating circuit being reset in response to the synchronizing signal, the triggering system determines the timing at which the triggering signal is output in accordance with image data of an image to be formed.
In this case, the triggering system may include a clock generator that generates a clock signal including clock pulses output at a predetermined frequency, a counter that counts the number of the clock pulses included in the clock signal, and a number setting system that sets a number to be counted by the counter, the triggering signal being output when the counter has counted the number set by the number setting system.
Optionally, the number setting system determines the number set to the counter based on the image data.
Further optionally, the triggering system may include a clock generator that generates a clock signal including clock pulses output at a predetermined frequency, a counter that counts the number of the clock pulses included in the clock signal, a number setting system that sets a number to be counted by the counter, the counter outputting a count-up signal when the counted number of the clock pulses reaches the number to be counted, and a delay unit that delays the count-up signal by one of a plurality of durations of time, each of the plurality of durations of time being shorter than a period of the clock pulses, the delayed count-up signal being output as the triggering signal.
According to another aspect of the invention, there is provided a scanning optical device having an automatic power control circuit for controlling an output power of a laser diode, provided with a detection system that detects an output power of said laser diode and outputs a value corresponding to the output power of said laser diode, a saw-tooth wave generating system that outputs a saw-tooth wave which varies as time elapses, a triggering system that outputs a trigger signal, and a sample-and-hold system that samples and holds the saw-tooth wave in response to the trigger signal. An automatic power control operation is performed based on said sampled and held value and the value corresponding to the output power of the laser diode.